Where does all the bad light end up?

1. Where does all the bad light end up? In a prism…

2. 10 June 2014 Lenses Objectives To understand the nature of the image produced in converging lens and to understand the uses of a converging lens in a camera.

3. ALL MUST be able to draw ray diagrams for converging and diverging lenses. MOST SHOULD be able to describe how to make converging lenses stronger. SOME COULD explain how lenses are used in cameras. OUTCOMES

4. What are lenses? Lenses alter the path of any light rays passing through them, refracting them toward or away from a single point. Lenses must be made of a transparent medium with a curved surface. It is the curve of the lens that determines the amount of refraction. There are two major types of lens, concave and convex, both of which have many uses in modern life.

5. Convex lenses A convex lens bulges in the centre. Convex lenses are also called converging lenses. This means that they refract light rays towards a single point. The point at which the rays cross over is determined by the distance between the lens and the light source. Uses for convex lenses include: spectacles for long sight camera lens magnifying glass eye

6. How do convex lenses work?

7. Convex lenses and parallel rays For light rays to be parallel when they enter the lens, the object must be an infinitedistance away. A convex lens will cause such parallel rays to converge at the focalpoint (F). The position of an object relative to this point is crucial when trying to establish the type of image formed by a convex lens.

8. Concave lenses A concave lens is one which tapers inwardsat its centre. Concave lenses are also called diverging lenses. This means the lens refracts light away from a single point. Uses for concave lenses include: spectacles for short sight How would a concave lens correct this eye’s short sightedness?

9. Focal length of a concave lens To find the focal length of a concave lens, rays parallel to the principal axis must be directed through it. As the rays diverge, we must trace their path backwards to find the point where they cross over. This is the focal point (F), while the distance from the focal point to the centre of the lens is the focal length (ƒ).

10. Investigating converging lenses Aim To help you to understand the behaviour of converging lenses. Method You can investigate the image formed by a converging lens using the equipment shown below. Put a brightly lit object at one end of a metre rule, with the lens in the middle. Use a piece of card with graph paper stuck on as a screen – move the card and the lens until the image on the screen is in focus. Investigate how the image changes as the position of the lens is changed. Results Analysis Plot a graph of magnification on the y-axis against image distance on the x-axis. Draw the line of best fit and work out the gradient.

11. copy and complete: Lenses work by changing the ……… of light because of refraction. Converging lenses make parallel rays converge to a ………. The point where the rays focus to is called the principle focus of the lens. A diverging lens makes ……… rays diverge (spread out). The ……… where the rays appear to come from is called the principle focus of the lens. The ………….. from the centre of the lens to the principle focus is called the focal length. direction point distance focus parallel Copy the diagram for a converging lens below. Copy the diverging lens and complete the diagram by showing how the light is spread out. Plenary

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13. Plenary Tweet what you have learnt today – (write a summery in 140 characters max)